It's just a little key. Somebody sent it in the mail. But the person it was sent to never got it - and nobody knows who sent it. So now it's in the Dead Letter Office at the city post office. When Danny hears about it, he decides to solve the ‘'BIG mystery of the LITTLE key.'' Danny and his friend Emma search for answers all over town. Who sent the key? Who was it for? And, what does it open?